How to Measure the Electrical Voltage Entering a Building using a VOM, DMM, or Voltage Detector Pen InspectAPedia® -
Basic introduction to use of simple instruments like neon testers, voltage detector pens, to detect the presence of live electrical voltage
Introduction to use of Volt-Ohm meters VOM's, Digital multi meters DMM's or multimeters to measure electrical voltage and amperage at a building
How to determine the size, capacity, or ampacity of electrical service at a building
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This article explains how to estimate the electrical service size, (or "electrical power" or "service amps") at a building by visual examination of the
service entry cables, electric meter and meter base, electrical service panel, main switch, and other details. Visual inspection and use of digital multimeters(DMMs), Volt-ohm meters (VOMs), neon testers, and electrical inspection safety are discussed.
Photographs and sketches illustrate electrical panels, meter bases, and electric meters.
One of the most frequently asked questions at ASHI Education Seminars and Conferences is "How do I determine the service amperage?"
It's not as difficult as one may think to get a reasonable handle on the electrical service capacity at a building without sophisticated analysis. But there are some pitfalls, and the process itself is dangerous. The articles from which much of this online material originated
appeared first in the ASHI Technical Journal, Vol. 2. No. 1, January 1992, "Determining Service Ampacity," Dan Friedman and Alan Carson,
and the ASHI Technical Journal, Vol. 3. No. 1, Spring, 1993, "Determining Service Ampacity - Another Consideration," Robert L. Klewitz, P.E.,
with subsequent updates and additions to the original text ongoing to 2/19/2006. Reprints of the originals and reprints of the Journal are available from ASHI, the American Society of Home Inspectors www.ashi.com
VOLTAGE by EQUIPMENT - Determining Electrical Voltage (240V/120V) using a neon tester, a VOM, DMM, Volt Meter, or Voltage Detector pen.
Instruments as simple as the neon-tester shown here or a digital multimeter (DMM), or an analog volt meter (volt-ohm meter) such as
those made by Triplett, Fluke, and Simpson can be used to measure the voltage level between the two power
feed lines to confirm the presence of 240 Volt service. (Some versions of this equipment can also be configured to measure amperage or amps).
Using a DMM, analog volt meter, or a neon tester, voltage between the two apparently "hot" wires
in the panel will confirm 240-volt service. If only one cable is "hot" (testing cable to ground) it is a 120 V system. If both cables are "hot" to ground but there is no voltage between them it is
a 120V system which has been "split" to look like 240V but it is not 240V. A volt meter is required to test for other voltage levels.
Safety Warning: The ASHI Standards of Practice and other home inspection standards for electrical inspections do not require the inspector to insert any instrument into the service panel. Therefore this testing is optional.
It's also a dangerous procedure and should not be undertaken unless the person conducting the examination is trained and competent to avoid electric shock. If the inspector is not trained for this procedure
s/he should never insert any instrument or tool into electrical equipment. See Safety Hazards and Safe Inspection
Procedures at Residential Electric Panels.
Here are photographs of the basic types of electrical test equipment useful for a building inspector, electrical inspector, contractor, or homeowner
A voltage detector of voltage sensor detects the presence of live AC voltage (some equipment also detects DC voltage), with a sensitivity typically between 5 and 1000 Volts. This equipment can sense the presence of live electrical devices or wiring without touching the device or wire.
Shown here (left) is the Greenlee® GT-16 adjustable voltage detector. Greenlee and other manufacturers recommend that you test the operation of the voltage detector on a known live circuit both before and after using it to test for the presence of electrical voltage at a wire or device. This device uses an AAA battery and has a sensitivity adjustment (the yellow dial in the center of the green patch on the device).
Some voltage sensors such as the FCB Voltage Pen operate without requiring a battery.
An ammeter measures electrical current. Our photo at left shows a multi-meter that can measure amps as well as serve ad a VOM - a volt-ohm volt meter.
A voltmeter measures the potential difference (voltage) between two points
An ohmmeter measures resistance. An ohm meter measures resistance in a circuit or on a component and is not normally connected to a circuit carrying live voltage.
A multimeter combines these functions into one piece of equipment and is often called a VOM or volt-ohm meter. Our little VOM shown at left is being used to check for voltage at an electrical receptacle. If you click and enlarge the photo you can see the meter reading.
A neon tester simply measures for the presence of live electrical power as we demonstrated above.
A continuity tester is a simple substitute for a more sophisticated ohm meter, and is used simply to confirm that there is electrical continuity on a circuit, that is,
that it is not interrupted by a switch or open connection.
Some simple ohm-meters or multimeters include a switch position that emits a tone if there is good electrical conductivity (low resistance) between the two points being touched by the probes.
Analog VOMs: A VOM or volt-ohm meter may also be found in an analog form such as our favorite and now collector-item little Jensen VOM that we've used for nearly 30 years. Here our photo shows the Jensen analog VOM with its probes attached but not connected to anything.
In 30 years of use we needed one repair of this instrument - an internal fuse which we couldn't find locally. On contacting Jensen Corporation, the company kindly sent us a few spare fuses. The total repair bill? $0.00.
Safety Warnings for Electrical Inspectors & Home Inspectors inspecting electrical services, wiring, or equipment
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The Original Authors: Alan Carson is an ASHI Member, national home inspection educator, author and building failures researcher in Toronto, Ontario.
Daniel Friedman, an original author of this article and the editor and producer of InspectAPedia where this article now appears is an ASHI Member, first ASHI Technical Committee chairman, editor and publisher of the ASHI Technical Journal, licensed home inspector, educator, and building failures researcher in Poughkeepsie, NY.
Robert Klewitz is a licensed professional engineer, a professional home inspector, an ASHI Member, and has served on the ASHI Technical Committee as well as in other ASHI activities. His practice is in Issaquah, WA.
Douglas Hansen, Robert Stead. Mark Cramer. Photographs: Daniel Friedman.
Critique, contributions wanted: Contact Us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution.
N. Srinivasan, MSEE, is a senior member of IEEE with 30 years experience in the electrical industry. Mr. Srinivasan is in Vienna VA.
Louis P. Babin generously contributed technical editing about the effects of doubling ampacity in an electrical circuit (September 2007)
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